1. Field of the Invention
The present invention relates to the delignification of raw material containing lignin and cellulose, such as wood, straw or other lignocellulosic material, while using lignin degrading organisms.
2. Description of the Related Art
It is known, e.g. from the U.S. Pat. No. 3,962,033, that it is possible to extract cellulose from wood and other raw material containing lignin and cellulose, by treating the raw material with an organism producing lignin degrading enzymes under conditions where the lignin is degraded without the cellulose being substantially affected. The organisms primarily intended in this patent are white-rot fungi, which normally produce both lignin and cellulose degrading enzymes. The patent is particularly directed to the use of the white-rot fungus Sporotrichum pulverulentum. According to the patent specification, treatment of the lignocellulosic material may be carried out by the addition of substances, primarily sugar, which inhibit production of cellulose degrading enzymes (cellulases) by the fungus so that mainly only the lignin degrading enzymes are active.
However, it is also possible to treat the raw material by using an artifically obtained mutant of the microorganism mentioned above, that has had its ability to form cellulases eliminated or reduced in relation to the organism present in nature. It is thus stated in the patent specification that suitable mutants of Sporotrichum pulverulentum may be obtained by ultraviolet irradiation or other mutagenic agents, and sorting out for pure growth the cellulase-deficient mutants thus formed.
It appears that Sporotrichum pulverulentum, as far as is known at present, is the best lignin degrading fungus. It has earlier been regarded as having an incomplete sexual cycle. Mutations of conidia (asexual spores) have therefore been resorted to for obtaining the cellulase-deficient mutants. It is known that basidiomycetes generally have two ways for their reproduction and survival, i.e. the sexual and the asexual form. The sexual form is characterized by the production and release of basidiospores, whereas the asexual form may involve the production of asexual spores (oidia, conidia, chlamydospores, arthrospores) which can be spread or remain in situ.
The sexual form naturally means a much larger genetic variation within the population than the asexual form.
It has now been found, however, that the cellulase-deficient mutants of Sporotrichum pulverulentum obtained in this way unfortunately have a substantially reduced ability to degrade lignin, compared with the wild strain. In any case, it has not been possible to obtain mutants in this way that degrade lignin (or delignify wood) to an extent sufficient for commercial utilization. This appears to be connected, inter alia, with these mutants not only being cellulase deficient but also xylanase deficient. For this reason little or no sugar is formed, which the fungus needs for producing H.sub.2 O.sub.2, which is inescapably required for the lignin degradation.